Sure the billionaires can leave CA. No loss there, because their money will stay there.
By "there" you mean "the money will remain in California". To an extent, yes. You can live in Austin and found a company where most of the workers are in San Jose. That's just inefficient and a pain. It makes it more likely that, at the margin, a founder will decide Austin is good enough, and hire there.
And billionaires themselves pay jack shit in taxes,
Can't let that slide. Wealthy people pay the lion's share of income and capital gains taxes, both federally and in California. They pay both more dollars and a higher proportion of income than poor people. The myth that billionaires pay little in taxes is just that, a myth.
FICA, sales, and property taxes may be a different story. There are instances where a rich person pays little but those are the exceptions, not the rule. California in particular is very vulnerable to stock market downturns because capital gains taxes plummet and Sacramento is left with an enormous budget hole.
It gets a bit murky when you start talking about who actually pays corporate income taxes. Some part of it is paid by consumers through higher prices, some by employees through depressed wages, and some by investors through lower dividends and capital gains. I don't think there's general agreement who pays what share of corporate income taxes.